The casting of articles such as electrical or electronic components that must have good insulating properties is normally carried out by means of a system having at least one mixing container, a dosing pump connected downstream thereto, a mixing chamber connected to the outlet of the pump, and a plurality of molds connected to this mixing chamber.
Reference may be had, in addition, to the references cited or made of record in the above-identified copending applications including U.S. Pat. Nos. 3,847,525 and 2,781,547; German Patent No. 1,272,524 and reissue U.S. Pat. No. Re 28 721 which may be material to the subject matter thereof and one of which is discussed in greater detail below.
In considering the background of the invention, a clear distinction should be made between the problems involved in dealing with the molding of thermoplastic synthetic resins, i.e. synthetic resins which harden by cooling and can be brought to a flowable state by heating, and those which involve thermosetting synthetic resins which are frequently referred to outside the United States as "duroplastics."
Synthetic resins of the latter type generally consist of two or more components which are intimately mixed prior to use and which react in a period referred to as the "pot life" to form a hardenable or hard mass which no longer is flowable.
Thermosetting resins include, for example, epoxy resins in which one component can include an epoxy compound while the other is an amine hardener and urethane-type compounds in which one component can be a prepolymer of characteristic composition and/or a di- or polyisocyanate while the other component can include diol or polyol compounds.
When thermosetting materials are handled, a significant danger is the setting or hardening of the mixture before the latter enters the mold. In other words if a portion of the mixture exceeds its pot life before entering the mold cavity, it will undergo reaction to set in portions of the apparatus upstream of the mold cavity.
The several components of the resin and its filler are normally kept in several respective containers in which they are premixed and wherein they may also be degassed by vacuum if necessary. The premixed components are fed by respective pumps in predetermined ratios to a single main mixing chamber through respective pressure-control valves having bypass lines that lead back to the premixing chambers or containers so that excess material can be fed back to these chambers. From the main mixing chamber the material is fed to the mold or molds.
The main disadvantage of this system is that the pressure with which the material is fed into the molds is the same for all of the molds, normally varying sharply pulse-wise. If all the molds do not have cavities of the same shape and volume, some of the cavities will fill up more rapidly than the others, or others will not be filled completely. Thus the pressurization must be maintained to fill even the largest cavities and compensate for resin shrinkage therein. What is more the recirculation of premixed material back to the storage chambers or containers can lead to clogging of the system if this material had had time to cure before it is recirculated.
In another known system synchronously operated pumps feed the various components from their storage vessels to a common mixing container, whence the mass is forced directly into the mold or molds. These pumps, which are well upstream of the mold, are therefore responsible for filling the mold and maintaining pressure therein during curing of the resin in the mold. As the gelling or hardening time inside the mold is at least several minutes long, these pumps must operate continuously to maintain the necessary pressure.
The result of such a system is premature wearing-out of the pumps.